Coatings comprising chromium (VI) oxides are known to inhibit corrosion of metal surfaces. Chromium (VI) oxides are the active species in so called chromium conversion coatings; i.e. they serve as a sacrificial cathode. Chromium conversion coatings are formed by the deposition of chromium (VI) species along with a chromium (III) species on the surface of a metal substrate, for example, on the surface of an anodized aluminum alloy. While the aluminum oxide layer of anodized aluminum is substantially inert, the aluminum oxide layer is porous and must be sealed to provide optimum protection of the underlying aluminum substrate. Chromium (VI) conversion coatings provide excellent "seal coats" for anodized aluminum surfaces. Although chromium (VI) has excellent corrosion inhibiting characteristics, it is also a known carcinogen. Current EPA and OSHA regulations require the chromium content of drinking water to be less than 100 micrograms per liter. Further restrictions on usage of chromium (VI) salts in industrial operations are anticipated. There have been significant research efforts directed to development of new industrial processes to enable more efficient usage of chemical reactants and having by-product/effluent streams with reduced environmental impact. The present invention is directed to a corrosion inhibiting composition and method that enables chromium (VI) based corrosion inhibiting sealant functionality using very low concentrations of chromium or by eliminating chromium (VI) and replacing it with an organic acid.
Thus, there is provided in one embodiment of the present invention an aqueous composition for inhibiting corrosion of metal surfaces. The composition comprises permanganate and a very low concentration of chromium (VI). The concentration of chromium (VI) in the aqueous solution is less than about 100 micrograms per liter, and the concentration of permanganate is about 0.2% to about 20% by weight of the aqueous solution. Surprisingly, the solution can be used to provide metal/metal oxide sealant functionality comparable to chromium (VI) conversion coatings utilizing much higher chromium concentrations.
In another embodiment of the present invention, a composition for inhibiting corrosion of metal surfaces includes permanganate and acetic acid. The concentration of acetic acid in the aqueous solution is between about 0.2% and 2.0% by volume. The concentration of permanganate is about 0.2% to about 20% by weight of the aqueous solution.
Other embodiments of the invention include methods of using the present sealant compositions and the resulting corrosion inhibited metal substrates. In one embodiment, the inhibited metal is the inner surface of a conduit of a recirculating heat transfer system, and the sealant solution is used as the heat transfer medium. In yet another embodiment of the present invention there is provided a kit for preparing a corrosion inhibiting solution for metal treatment. The kit comprises a water soluble compound of chromium (VI) and a water soluble permanganate salt. The weight ratio of chromium (VI) to permanganate in the kit is about 1:2.times.10.sup.4 to about 1:2.times.10.sup.8. A similar kit can be provided for preparing the acetic acid solution.